How can I use my own physics engine?

Let's suppose I have my own physics engine, how can I make it interact with gazebo? I can't find any description on how the simulation loop works or when all the physic routines are called. Looking at the API I can see there is a section "Physics" and how the other engines reimplement these classes and methods, what I want to know is how can I modify the physical behavior of the various simulation element at various level and moment? thanks in advance for any answer